My dear, please understand that I am not blaming or criticizing you, I am simply stating a fact based on what you have told us. You are obviously not competent enough to care for this little bird. I know you have the best of intentions and I know that you are trying very hard but, obviously, you are not been successful and the baby cannot go without enough food for long. Handraising a baby bird is not something that you can learn as you go along, you need to know what you are doing and if you were doing it right, he wouldn't be screaming and screaming. Taking him out for a walk doesn't do it and doing 'ladders' is not even recommended for adult birds (it's a flooding technique) much less a baby. If he doesn't eat enough or the right food, it will weaken and die. He is screaming because he is hungry, lonely and scared. This is not a little bird that was born in captivity and is used to people, he is a baby bird that was taken from his parents and he misses them, his siblings and the food that he is used to getting. PLEASE, I BEG YOU to contact these people
http://bwcindia.org/. They will help you. It's illegal in India to trap, keep, sell or buy indigenous birds (the vet should have told you that!) - it's even illegal to cage them.